Inliners International Forums General Discussion Engines GMC SBC valve swap

I am building a GMC with a 302 “D” port 2193983 casting head. As widely recommended I am planning on installing SBC 1.94” intake and 1.6” exhaust valves +0.100 with SBC valve springs. I had read the Z/28 spring will work in this application but they are only 1.250” in diameter, too narrow for the approx. 1.440” diameter spring seat in the head. That is much more wiggle room than I am comfortable with. I chose to use PBM #3400 dual spring w/damper valve springs. The GMC installed height spec of 1.821” is close to the Z/28 spring spec of 1.850”. With a slight cleanup of the seat to 1.440” diameter the PBM springs fit in the pocket perfectly. However, the installed height measured from the pocket to the bottom of the retainer with the 5.025” long valve is about 2.030”. How can the valve closed spring height be off by 0.21”? The damper isn’t even under compression at that height. What am I missing here?

Greetings . . . on the stovebolts the valve spring rides on a separate removable seat - not directly on the head:
Chevy Parts Master '29 - '54
specifically part 0.311. I suspect the jummies have a similar part. This will close some of the gap.

Also, I am running McGurk hold downs and a custom after market spring package. I needed to shim underneath the stock spring seats to get the right 'installed height' and corresponding seat pressure:
Inliners Post circa '14
Lucky for me Frank McGurk included the shims in the Hold Down and Inner Spring package I am using.

Substituting valves and springs you will likely have to shim to get the installed height right.

regards,
stock49

I have one 270 and two 302 GMC heads and none of them have a removable seat and none are shimmed. 0.21" is a pretty thick shim. But then I doubt any of them are in their original as built condition either.

I just checked the 47-54 GMC Maintenance Manual. The valve train illustration does not show a valve pocket locator or shims like the Stovebolt uses.

Indeed . . . .210" is long way without a spring locator taking up some of the slack. Are you using the stock Jummy hold downs or SBC ones?

I wonder if this valve swap requires a another annulus to be cut lower down on the valve stem?

No experience here.

The Jummy head is cast with a 1/8" deep valve pocket the width of the valve spring to locate the spring so no locaters are necessary. Everywhere I have read says to use a SBC valve spring +0.100" in length ... which is half the 0.21" it seems to be too long. If I had used a SBC spring at -.100" I would be right on the installed height. I have reinstalled and measured the old valve and spring and it is about the same as the new ones. I am wondering if the previous owned ran into the same issue and just went with what it was. My concern is the spring pressure at 2.030" height may be way too low. The mfgr. spec is 120# at the valve closed height of 1.850" and 320# at 1.200" but it would never get that high. It will be much lower at 2.030". The free uncompressed length of the spring is 2.33" so it is only compressed 0.30" as it is installed.

I've used .100 long SBC valves in several GMC heads before because I was able to retain the correct valve length when machining for OS valves, but more importantly, you need to use the length that retains the correct valvetrain geometry or you'll eat up rocker tips or valve tips.

Wouldn't the OS valve still seat in the same position (depth)as the stock valve when the seat is opened up? If so wouldn't the stock valve length still be correct for the new valve geometry?

When you machine the seats for OS valve and cut the seat to the same depth as the old valve, you are essentially sinking the valve and hurting flow potential. So no, you wouldn't always expect the OS valve to have the same depth of cut as the original. Also by doing so, you are assuming the depth was correct to begin with and it may not be. These old heads could have had 10 or more valve jobs done in its lifetime, so it could be far from the standard you need to compare it too.

Another problem with swapping newer technology hardware into the older engines is that many of the old engines have tips on the valves that were much longer than the modern SBC valves, and even using a +.100 longer valve may help on the spring installed height, it still may not get you were you need to be geometry-wise because of the plane the valve tip it is in is still too far away from where it was originally. There is definately a balancing act between all of these parameters to find a happy medium.

It sounds like your seats are recessed too deep - I have standard SBC valves in my 270 head and I altered the height of the rocker arm to get the valve geometry correct.



I ended up lowering the rocker shaft about 0.050 inches I believe (can't remember) and used Crower 68301-1 springs

68301-1 (per machine shop measurements)
Spring Rate 378 lbs/inch
Free Height 1.98 inches

Installed Height1.77 inches
Open Height 1.34 inches

Closed Pressure 90 lbs
Open Pressure 250 lbs

The motor has been to 5000 rpm several times with no valve float and when I removed the head and cam at 20,000 miles, the valves and seats looked great.

I think you may be right on the valve seat depth being too deep although I have measured the installed height with the 1.94" valve in the existing unrelieved port and it is still long. To compound things the spring seat pockets in the head are spec'd different with the exhaust being 0.070" deeper in the head without any change in the corresponding installed height. Unfortunate I haven't found a machinist here in S FL who even recognizes a Jummy when he see's one. The last one I showed it to thought it was a MBZ diesel! Needless to say I packed it up and took it back home.